Magnetic sensor systems are increasingly important in various industries. For instance in the automotive industry, various sensor systems, such as parking sensors, angular sensors e.g. in throttle valves, ABS (Automatic Braking System) sensors and tire pressure sensors are found in modern vehicles for improving comfort and safety. Magnetic sensor systems are particularly important in automotive applications, because magnetic fields penetrate easily through most materials. In addition, magnetic sensors are highly insensitive to dirt, unlike for example optical sensors.
Several different magnetic sensor technologies are currently available, such as sensors based on the Hall effect, lateral magnetic field sensors based on silicon and constructed on the basis of bipolar lateral magnetoresistors (LMRs), lateral magnetotransistors (LMTs), and lateral magnetodiodes (LMDs) as well as sensors based on the magnetoresistive effect, such as anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) sensors. Hall effect based sensors and the bipolar lateral magneto-resistors, transistors and diodes, i.e. LMRs, LMTs and LMDs, rely on the Lorentz force caused by the magnetic flux acting on moving charge carriers. The sensing principle of AMR and GMR sensor systems is based on the physical phenomenon that the electric resistance of a ferromagnetic material depends on the angle between the magnetization and the direction of the electric current within an AMR or GMR sensing element.
Silicon-based magnetic sensors which are sensitive for magnetic field (H) or flux-density (B) components in the plane of a chip can be constructed in plural ways, for example as bipolar magnetoresistors (MRs), magnetotransistors (MTs) and magnetidiodes (MDs), each comprising two or more current-collecting contacts (collectors) and at least one current-emitting contact (emitter) arranged in between the collectors. MTs have a base contact in addition to the emitter and collector contacts and have at least one pn-junction between an emitter and a collector. MDs also have at least one pn-junction between an emitter and a collector, like MTs, but do not have a base contact, unlike MTs. MRs have no pn-junction between an emitter and a collector.
On SOI (silicon on oxide) substrates, the contact structures are made as vertical or lateral magnetotransistors (VMTs or LMTs, respectively), lateral magnetodiodes (LMDs) or lateral magnetoresistors (LMRs). Using SOI substrates has the advantage of preventing leakage currents that would be present in such sensors when made in bulk CMOS (complementary-symmetry metal-oxide-semiconductor) process technology.
The operation of lateral magneto (transistor, resistor and diode) sensors relies on the substantially symmetric geometry of the emitter-collector-contact structure and the fact that the emitter current is split in two components having opposite directions in the space between the collectors, and is influenced by the magnetic flux density (B) through the Lorentz force acting on the two split current portions in two opposite directions. Accordingly, the differential collector current is a measure for the magnetic flux density (B). The splitting of the emitter current suffers from an imbalance in the resulting collector currents even when the magnetic flux density B is zero. This difference of collector currents is referred as the “offset” of the sensor. Even emitter-collector-contact structures with substantially perfect geometric symmetry design suffer from offset (and offset spread).
One possible cause for the offset may be the presence of surface (shallow) trench isolation areas (called STI) between p+ and n+ areas used for collector and emitter contact structures (or for the contact areas associated with these functions). The strain and stress (and charged) interface states associated with these STI areas may be a source of the imbalance between the collector currents due to the statistical nature of the imperfections induced by the STI processing, whereby these imperfections are not all the same and are not equally or symmetrically distributed in the STI areas.
Other causes of the offset may be related to mask misalignment, to non-uniform doping distributions, to mechanical stresses and to thermal gradients. Since it is very difficult to fabricate devices that are insensitive to all these causes, one has to do anything possible to make the devices symmetrical, in layout, in doping distribution, etc. The doping distribution in a standard process is not always ideal. For example, implantation is often performed under a tilt angle of the substrate. The doping symmetry may be improved by making such implantation four times (quad), whereby each time the substrate is rotated by 90 degrees. However, if this is not possible, a systematic offset remains.
The object of the present invention is to provide silicon, preferably SOI, -based lateral magnetic field sensor systems, which have substantially symmetrically layouted emitter-collector-structures and involve a splitting of the emitter current in collector-related components of mutually opposite directions, in which the offset and eventually the offset of the differential current spread is reduced or even cancelled to zero.